The invention relates to an assembly for forming a mechanical connection to an object, to a method of forming a mechanical connection to an object, and to an object to which a mechanical connection has been made.
It is known to make a mechanical connection to an object by attaching a collar to the external surface of the object, through which a force can be applied to the object in a direction parallel to the surface. This connection technique is used in particular to connect elongate objects which have a constant cross-section to one another, for example tubes (which term shall include pipes) and rods. For example, U.S. Pat. No. 3,484,123 discloses a tube coupling assembly comprising a collar which is rigidly fixed to a first tube by swaging, and a union member which is rigidly fixed to the second tube, also by swaging. The union member extends beyond the end of the second tube and has a threaded external surface. The assembly includes a nut which has a radially inwardly projecting flange at one end which abuts the collar, and which is threaded internally at its other end for engaging the threaded surface of the union member, so that rotation of the nut relative to the union member forces the two tubes towards one another.
It is also known to fix a collar to an object by welding instead of swaging. Welding is particularly useful when the material of the object makes swaging difficult, as is in the case with tubes formed from titanium.
More recently, it has been proposed to use a collar formed from a shape memory alloy, in order to avoid the need to swage an object or to form a weld for joining the object to the collar. Shape memory alloys exhibit a shape memory effect as a result of their ability to transform between martensitic and austenitic phases. The transformation may be caused by a change in temperature: for example, a shape memory alloy in the martensitic phase will begin to transform to the austenitic phase when its temperature increases to a temperature greater than A.sub.s, and the transformation will be complete when the temperature is greater than A.sub.f. The reverse transformation will begin when the temperature of the alloy is decreased to a temperature less than M.sub.s and will be complete when the temperature is less than M.sub.f. The temperatures M.sub.s, M.sub.f, A.sub.s and A.sub.f define the thermal transformation hysteresis loop of a shape memory alloy. An article may be formed in a desired configuration while in its austenitic phase. If it is then cooled so that it transforms to the martensitic phase, it can then be deformed by up to about 8%. The strain imparted to the article is recovered when the article is subsequently heated so that it transforms back to the austenitic phase. Further information is available in the article by L. M. Schetky in Scientific American, Volume 241, pages 68 to 76 (1979) entitled Shape Memory Alloys.
The use of a shape memory alloy collar has the advantage that significantly less time and less skill are required on the part of the installer than are required to affix a collar either by swaging or by welding. Furthermore, the outlay in equipment required to install a shape memory alloy collar is significantly less than that required to install a collar by welding or swaging.
It is generally important that the collar be positioned accurately relative to the end of the object to which it is to be affixed. Manipulation of the collar can be difficult when the collar is small, or when the collar is formed from a shape memory alloy and the temperature of the collar must be increased to cause it to shrink transversely, whether by removal of the collar from a cryogenic fluid, or by the application of heat to the collar from an external source; the use of tools or insulation gloves can then make manipulation of the collar particularly difficult.